I have developed EvoX jointly using gcc 2.7.2 on Suns (OS 4.1.3) and Metrowerks CodeWarrior 8 on the Mac, so the EvoX base should be reasonably easily portable to new platforms. Of course whether a given simulation will run on a different platform will depend upon the classes in that simulation being compilable on that platform (normally no problem).
Some of the graphical analysis facilities of EvoX require Plplot. This was designed to be highly portable (requiring little more than the ability to draw lines and dots in its most basic form) graphics library, so all that is actually needed is a new display driver (hey, plplot comes with about 20 of these, so they can't be too hard to write!). Unfortunately the Tcl/Tk-Plplot interface is dependent upon X-windows, so that aspect of plplot is not currently portable. What is needed is an update to the Mac and Windows plplot drivers to make them compatible with the Xwin driver.